Abstract: A pollution or emission control system for retrofitting on older vehicle engines includes a catalytic converter with an in-built oxygen sensor for mounting in the exhaust line, and a supplemental air input line containing a control valve for connection to the engine intake manifold to control the amount of air supplied to the engine. A computer monitors the oxygen sensor output and the engine speed, and controls opening and closing of the control valve according to the detected conditions. The computer also controls the speed at which the valve is opened or closed dependent on the engine speed and optionally the engine temperature, so that the valve can be closed faster at slower engine speeds and opened faster at higher engine speed. The computer is additionally capable of producing diagnostic telemetry, both digital and analog, to assist installation of the invention and test for proper operation once installed.